This invention relates generally to electrical control systems, and more specifically to an aircraft electrical control system which disconnects power to a load when a current imbalance is sensed.
In the electro-mechanical arts, current imbalances are indicative of serious problems that can lead to disastrous results, such as arcing within fuel pumps. Since fuel pumps are often housed within a fuel vessel to directly pump fuel out of the vessel, arcing within a fuel pump can lead to an explosion of fuel-air mixture and a subsequent breach of the fuel vessel, which can be catastrophic. In light of the seriousness of such an event, a device or methodology is needed which can suppress this type of arcing, as well as other associated problems. Presently, a common type of circuit protection device being utilized in aircraft is a thermal circuit breaker. However, arcing typically does not cause thermal circuit breakers to activate. Thus, there has been a long-felt need for the function of current imbalance detection in an aircraft. One very important form of current imbalance is a ground fault in which current is flowing between a circuit or electrical device to ground, when such current flow is not desired. In the prior art, ground fault detection has been addressed by a separate ground fault interruption unit. However, such prior art systems have had limitations, including the necessity of rewiring the aircraft. In addition to the requirement to rewire the aircraft, additional space had to be found to accommodate the ground fault interruption system.
One currently available ground fault interruption unit made by Autronics (model 2326-1) has been used in large commercial aircraft for the purpose of ground fault protection for fuel pumps. The Autronics unit detects a ground fault and outputs a signal indicative of a fault by use of a current transformer and acts by removing power to the fuel pump control relay.
Two different types of ground fault conditions can occur. The first type of ground fault condition occurs during normal ground fault interruption (GFI) operation (Control Voltage On state), in which the ground fault occurs after the relay contacts are closed, and power is being provided to the load. In relay terminology, this is known as “carry-break,” i.e., the relay contacts are fully closed and carrying the current when a contact has to interrupt the fault current. Many conventional 25 Amp relays can interrupt as much as a 500 Amp current without failure. However, the second type of ground fault condition is a more severe condition for relay contacts. This second type of ground fault condition occurs when the GFI is just powered and the relay contacts close into an existing ground fault. In relay terminology, this is known as “make-carry-break,” i.e., the relay contacts have just closed and the relay is immediately commanded to open and interrupt the fault current. In this second type of ground fault condition, severe arcing can occur at the relay contacts as a result of “contact bounce,” in which moving contacts make and break electrical contact several times due to their inherent mass and springiness before the contacts come to rest. Unfortunately, severe arcing can occur during contact bounce in this type of ground fault condition which can also result in the welding together of the relay contacts. Tests performed with relays have shown that in this second type of ground fault condition, 25 Amp conventional relay contacts can become welded together with an arcing ground fault current as low as 150 Amps.
A need therefore exists for an improved circuit protection device for aircraft that can handle both carry-break and make-carry-break types of ground fault conditions, and fault current amplitudes, so as to insure that ground fault interruption can always be carried out without failure of the ground fault circuit interrupter. It would also be desirable to provide a ground fault circuit interrupter that at power up will perform a test to confirm that none of the relay contacts have failed in a closed position, and to check that the switching FET which controls the relay coil has not shorted.
It would further be desirable for the circuit protection device to be included within an existing device in the aircraft, or to be packaged with an existing device, sharing the same connections to existing electrical circuits, since space for avionics is limited in any aircraft and adding wiring to accommodate a new device is very difficult. The present invention addresses these and other concerns.